In reply to kierop:
I would always prefer outdoor piste and off-piste if possible (just as I would prefer 'proper' climbing to the wall if that was a practicable option). Years ago I used to go from Manchester to the Sheffield slope every week.
For me, dry-slope was a good way to practice technique, and keep a degree of ski-fitness, (and still good fun) although 'dry-slopers' tended to skid rather than carve turns. More useful if working on specific things rather than repeated blasting down
The first runs back on dry slope after being on snow were always disappointing as the dry slope felt slow and sluggish.
As natehd9 said above, quite a bit of UK freestyle talent developed on the snowflex slope at Halifax.
Interesting to note that the dry slope at Ski Rossendale is still going reasonably well, despite being within an hour of the Chill Factore indoor snow slope at Trafford Park.
If going regularly, cost is a big factor - for an adult, an annual pass for unlimited entry at Rossendale is £32.50 per month (£390 total), whereas membership at Chill Factore is £100, but only gives a 30% discount on entry prices (£26 for 2 hrs off-peak, £32 for 2hrs peak) - so, at 13 visits a year they cost about the same but with more use the 'cost per visit' to Rossendale gets cheaper, while CFe remains at the same rate.
Whilst I do go to CFe at times, I'm not enamoured with the feel of the place, whilst I understand they are costly to run - the whole place has a theme-park feel, where the focus is more about 'increasing footfall and consumer spending'. Rossendale feels more of a community facility for skiers/boarders.